1. Field of the Invention
The present invention relates to a specimen inspection instrument for inspecting a specimen by impinging the specimen with a primary beam such as an electron beam or laser beam and, more particularly, to a specimen inspection instrument capable of detecting a very weak electrical current flowing through a specimen.
2. Description of the Related Art
In a scanning electron microscope, a sharply focused electron beam is directed to a specimen and scanned across it. Secondary electrons or reflected (back-scattered) electrons produced from the specimen are detected with a detector. A detector output signal obtained thereby is sent to a display device to produce a secondary electron image or reflected electron image of the specimen. In another conventional procedure, electrons absorbed by the specimen and flowing through the specimen toward ground potential are detected as a specimen current, and the detected current is sent to a display device to display an electrical current image of the specimen.
Where a specimen current is detected, the current of the primary electron beam impinging on the specimen is usually as small as on the order of picoamperes. Therefore, a weaker current must be detected. This requires that the instrument can prevent mixing of noises caused by electromagnetic waves from its surroundings. Consequently, the specimen is placed within a specimen chamber having an electromagnetic shielding function.
Although the specimen chamber having an electromagnetic shielding function is capable of shielding the specimen from electromagnetic waves from its surroundings, a positioning mechanism placed within the specimen chamber for moving the specimen includes a drive motor. This motor can be a source of electromagnetic waves. Consequently, it is difficult to precisely amplify the specimen current at a high amplification factor. Especially, where the specimen is large, it is inevitable to use a large-sized specimen-moving mechanism. As a result, intenser electromagnetic waves are emitted, producing greater effects.
In view of the foregoing, the present invention has been made. It is an object of the present invention to provide a specimen inspection instrument permitting precise detection of specimen currents.
A first embodiment of the present invention, which achieves this object, lies in an instrument for inspecting a specimen by impinging a primary beam onto a surface of the specimen insulatively held to a specimen-moving mechanism mounted within a specimen chamber, detecting the resulting electrical current through the specimen with a detector, and inspecting the specimen based on the thus obtained detector output signal. This instrument is characterized in that it has an amplifier having a reference input terminal and placed outside the specimen chamber to amplify the detector output signal, a first lead wire for passing the detector output signal to the outside amplifier through the wall of the specimen chamber, a conductive partition member insulatively inserted between the specimen and the specimen-moving mechanism to form a partition electrically isolated therefrom, and a second lead wire for connecting the conductive partition member with a conducting member and with the reference input terminal of the amplifier. This conducting member is placed around the portion of the first lead wire passing through the wall of the specimen chamber. The reference input terminal determines a reference potential for the output from the amplifier.
A second embodiment of the present invention lies in an instrument for inspecting a specimen by impinging a primary beam onto a surface of the specimen insulatively held to a specimen-moving mechanism placed within a specimen chamber, detecting the resulting electrical current through the specimen with a detector, and inspecting the specimen based on the resulting detector output signal. This instrument is characterized in that it has a differential amplifier having a reference input terminal and placed outside the specimen chamber to amplify the detector output signal, a first lead wire for passing the detector output signal to the outside differential amplifier through the wall of the specimen chamber, a second lead wire for connecting the specimen-moving mechanism with a conducting member and with the reference input terminal of the differential amplifier, a conductive specimen holder acting to hold the specimen and insulatively held by the specimen-moving mechanism, and a third lead wire for passing the signal from the specimen holder to a second input terminal of the outside differential amplifier through the wall of the specimen chamber. The conducting material is placed around the portion of the first lead wire passing through the wall of the specimen chamber. The reference input terminal determines a reference potential for the output from the amplifier.
A third embodiment of the present invention lies in an instrument for inspecting a specimen by impinging a primary beam onto a surface of the specimen, detecting the resulting electrical current through the specimen with a detector, and inspecting the specimen based on the obtained detector output signal. This instrument is characterized in that an antenna is placed close to the specimen and that the difference between the signal from the antenna and the current through the specimen is taken as the detector output signal.
A fourth embodiment of the present invention lies in a specimen analysis instrument for inspecting a specimen by impinging a primary beam onto a surface of the specimen insulatively held to a specimen-moving mechanism placed within a specimen chamber, detecting an electrical current induced across the specimen with a detector, and inspecting the specimen based on a resulting detector output signal. This instrument is characterized in that it has a differential amplifier for amplifying said detector output signal, said differential amplifier having input terminals, a first lead wire for passing the detected electrical current flowing through the specimen to said differential amplifier, an antenna mounted close to said specimen, a second lead wire for passing a signal from said antenna to said differential amplifier, and a selector means for switching the input terminals of said differential amplifier between a grounded state and an ungrounded state.
A fifth embodiment of the present invention lies in a specimen analysis instrument for inspecting a specimen by impinging a primary beam onto a surface of the specimen insulatively held to a specimen-moving mechanism placed within a specimen chamber, detecting an electrical current induced across the specimen with a detector, and inspecting the specimen based on a resulting detector output signal. This instrument is characterized in that it has a differential amplifier for amplifying said detector output signal, said differential amplifier having input terminals, a first lead wire for passing the detected electrical current flowing through the specimen to said differential amplifier, an antenna mounted close to said specimen, a second lead wire for passing a signal from said antenna to said differential amplifier, and a single-input amplifier for amplifying the detected current taken out via said first lead wire.
Other objects and features of the present invention will appear in the course of the description thereof, which follows.